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Laser Truss Sensor for Segmented Telescope Phasing

Thursday, 22 September 2011

A paper describes the laser truss sensor
(LTS) for detecting piston motion
between two adjacent telescope segment
edges. LTS is formed by two point-topoint
laser metrology gauges in a
crossed geometry.

A high-resolution (<30 nm) LTS can
be implemented with existing laser
metrology gauges. The distance change
between the reference plane and the target
plane is measured as a function of
the phase change between the reference
and target beams. To ease the bandwidth
requirements for phase detection electronics
(or phase meter), homodyne or
heterodyne detection techniques have
been used.

The phase of the target beam also
changes with the refractive index of air,
which changes with the air pressure, temperature,
and humidity. This error can
be minimized by enclosing the metrology
beams in baffles. For longer-term
(weeks) tracking at the micron level
accuracy, the same gauge can be operated
in the absolute metrology mode with
an accuracy of microns; to implement
absolute metrology, two laser frequencies
will be used on the same gauge. Absolute
metrology using heterodyne laser gauges
is a demonstrated technology. Complexity of laser source fiber distribution can
be optimized using the range-gated
metrology (RGM) approach.

Question of the Week

This week's Question: This month, the Federal Aviation Administration proposed long-awaited rules on the commercial use of small drones, requiring operators to be certified, fly only during daylight, and keep their aircraft in sight. The ruling,...